An imager, for example, a complementary metal oxide semiconductor (CMOS) imager, includes a focal plane array of pixel cells; each cell includes a photo-conversion device, for example, a photogate, photoconductor or a photodiode overlying a substrate for producing a photo-generated charge in a doped region of the substrate. A readout circuit is provided for each pixel cell and includes at least a source follower transistor and a row select transistor for coupling the source follower transistor to a column output line. The pixel cell also typically has a floating diffusion node, connected to the gate of the source follower transistor. Charge generated by the photo-conversion device is sent to the floating diffusion node. The imager may also include a transistor for transferring charge from the photo-conversion device to the floating diffusion node and another transistor for resetting the floating diffusion node to a predetermined charge level prior to charge transference.
FIG. 1 illustrates a block diagram of a CMOS imager device 101 having a pixel array 100 with each pixel cell being constructed as described above. Pixel array 100 comprises a plurality of pixels arranged in a predetermined number of columns and rows. The pixels of each row in array 100 are all turned on at the same time by a row selected line, and the pixels of each column are selectively output by respective column select lines. A plurality of rows and column lines are provided for the entire array 100. The row lines are selectively activated in sequence by the row driver 158 in response to row address decoder 157, and the column select lines are selectively activated in sequence for each row activated by the column driver 155 in response to column address decoder 154. Thus, a row and column address is provided for each pixel.
The CMOS imager is operated by the timing and control circuit 156, which controls address decoders 157, 154 for selecting the appropriate row and column lines for pixel readout, and row and column driver circuitry 158, 155 which apply driving voltage to the drive transistors of the selected row and column lines. Pixel output signals typically include a pixel reset signal, Vrst, taken from the floating diffusion node when it is reset and a pixel image signal, Vphoto, which is taken from the floating diffusion node after photo-generated charge representing an image is transferred to it. Vrst and Vphoto are read by a sample and hold (S/H) circuit 153 and are subtracted by a differential amplifier 152, which produces a differential signal, Vrst−Vphoto, for each pixel cell, which represents the amount of light impinging on the pixel cells. This differential signal is digitized by an analog to digital converter 151. The digitized pixel signals are then fed to an image processor 150 to form a digital image.
Imager pixel cells, including CMOS imager pixel cells, typically have low signal to noise ratios and narrow dynamic range because of their inability to fully collect, transfer, and store the full extent of electric charge generated by the photosensitive area of the photo-conversion device. The dynamic range of a pixel is commonly defined as the ratio of its largest non-saturating signal to the standard deviation of the noise under dark conditions. The signal representative of the photo-generated charge is proportional to the amount of charge collected by the photo-conversion device, and may be diminished if charge is lost during transfer or storage.
Another source of pixel cell error is called blooming. Blooming is caused when too much light enters a pixel cell and the pixel cell becomes saturated and unable to hold all of the charge generated by the photo-conversion device. Consequently, the excess photo-generated charge may overflow the pixel cell and contaminate adjacent pixel cells. The overflow of charge from one pixel cell to the next can create a bright spot or streak in a resultant image, called blooming. Anti-blooming gates have been developed to bleed off charge from a photo-conversion device to avoid contamination of adjacent pixel cells and the resultant error.
Since the size of the electrical signals generated by any given pixel in a CMOS imager are very small, it is especially important for the signal to noise ratio and dynamic range of the pixel cell to be as high as possible. Generally speaking, these desired features are not attainable, however, without additional devices that increase the size of the pixel cell. Therefore, there is a need for an improved pixel cell for use in an imager that provides high signal to noise ratio and high dynamic range while maintaining a small pixel size.